This invention relates to nuclear spin tomography in general and more particularly to an antenna device for exciting an at least largely homogeneous magnetic high-frequency field and/or for receiving corresponding high-frequency signals in a nuclear spin tomography apparatus.
An antenna device for use in nuclear spin tomography, with a tubular antenna part which is designed at least largely so as to pass low-frequencies for magnetic gradient fields, and consists of an electrically highly conductive non-magnetic material of predetermined thickness and is connected to an external energy feed or receiving device is described in DE-OS No. 31 33 432.
In the field of medical diagnostics, image-forming methods have been developed in which by calculation or measurement, integral resonance signals of nuclei of a given element of a body to be examined are analyzed, particularly of a human body or part of a human body. From the spatial spin density and/or relaxation time distribution to be obtained in the manner, an image similar to the x-ray tomograph can be constructed. Suitable methods are known under the designation "nuclear spin tomography" (Nuclear Magnetic Resonace tomography) or "Zeugmatography".
A requirement in nuclear spin tomography is a strong magnetic field which is generated by a so-called base field magnet, is as homogeneous as possible in a region of predetermined extent and into which the body to be examined is placed along an axis which generally coincides with the orientation axis of the magnetic base field. Superimposed on this base field are stationary and/or pulsed, so-called gradient fields. For exciting the individual atomic nuclei in the body to perform a precession motion, a special antenna device is further required, by which means of a high-frequency magnetic alternating field (RF alternating field) can be excited for a short time and which can also be used for receiving the RF signals connected thereto if a separate measuring coil is not provided for this purpose.
As is well known, the quality of the sectional images in such apparatus for nuclear spin tomography (NMR tomography) depends on the signal-to-noise ratio of the induced nuclear spin resonance signal. Since this signal-to-noise ratio in turn depends on the strength of the magnetic base field and increases with frequency, it is desirable to provide frequencies as high as possible for high base fields (see "Jour. Phys. E: Sci. Instrum.", volume 13, 1980 pages 38 to 44).
With the known RF antenna device mentioned at the outset, RF fields with high frequencies of about 20 MHz or more can be excited and recieved. To this end, the antenna device contains a tubular antenna part of electrically highly conductive nonmagnetic material. This antenna part represents and envelope around several conductor sections which form at least one pair of conductors which are disposed on an imaginary cylinder surface, around which the envelope is arranged concentrically at predetermined spacings. On the at least one conductor pair and the envelope, wave propagation with very high frequency is then made possible, resonance conditions being adjusted in such a manner that fields oscillating in the same phase are developed in the entire volume of interest in the form of standing waves on the pair of conductors. Since furthermore the common envelope around the pair of conductors is designed so that is passes, at least largely, low frequencies, the low-frequency gradient fields can accordingly propagate unimpeded in the volume into which the body to be examined is to be placed.
It is an object of the present invention to simplify this known antenna device.